Tricalcium phosphates, their composites, implants incorporating them, and method for their production

ABSTRACT

Methods for the synthesis of tricalcium phosphates are presented, as well as a series of specific reaction parameters that can be adjusted to tailor, in specific ways, properties in the tricalcium phosphate precursor precipitate. Particulate tricalcium phosphate compositions having an average crystal size of about 250 nm or less are provided. Compositions of the invention can be used as prosthetic implants and coatings for prosthetic implants.

FIELD OF THE INVENTION

The present invention generally relates to bioceramics, particularlytricalcium phosphate bioceramics, composites incorporating thesematerials, and methods for their production.

BACKGROUND OF THE INVENTION

There is a widely recognized need for an implant material that providesexcellent structural support for a variety of clinical applicationswhile providing for osteointegration over acceptable periods of time.Conventional metal implants are designed to ensure mechanical stabilityof the implanted region to meet short-term mechanical goals but raise anumber of longer-term clinical concerns including protuberance over theskin, non-uniform healing, bone atrophy, implant migration andloosening, all of which may lead to a second surgery to remove theimplant.

The morbidities associated with metallic implants have stimulatedinterest in polymeric and resorbable implants compromised of polylacticacid, polyglycolic acid, copolymers thereof, polymethylmethacrylate,polypropylenefumarate, collagen, or collagen-glycoaminoglycans. Thesedevices have not been widely accepted due to a number of clinicalcomplications associated with poor mechanical stability, formation ofsinus tracts, osteolysis, synovitis, localized inflammation, andhypertrophic fibrous encapsulation. As a result, a clinical demand forstronger, more biocompatible and resorbable orthopedic implants for usein both load-bearing and non load-bearing applications exists. Such animplant will incorporate a biomaterial possessing the followingproperties: 1) mechanical stability at the injured site for the requiredduration to allow adequate healing; 2) biocompatibility with thesurrounding host tissue; 3) osteointegration with the host bone; and 4)elimination of aseptic inflammation.

Bioceramics have been identified as biomaterial that may potentiallypossess the desired properties discussed above. They have foundwidespread use in craniomaxillofacial, dental, and orthopedicapplications as well as oral, plastic, and ear, nose, and throat surgeryand are categorized according to their in vivo interaction: bioinert,bioactive, and resorbable. Common bioceramics are alumina, zirconia,calcium phosphate-based ceramics, and glass-ceramic composites.

Bioinert bioceramics include alumina and zirconia, and are characterizedas such because the body recognizes them as a foreign object andencapsulates them in fibrous tissue. Furthermore, tissue growthassociated with this reaction is used to mechanically fix the inertceramic article within the body by encouraging tissue ingrowth intosurface irregularities or intentionally introduce porosity. Althoughmany ceramic compositions have been tested as implants to repair variousparts of the body, few have achieved human clinical application.Problems associated with these ceramics typically involve the lack of astable interface with connective tissue and/or a mismatch in mechanicalproperties between the implant and the tissue to be replaced (see Henchin “Bioceramics: from Concept to Clinic,” J. Am. Ceram. Soc., 1991, 74,1487-1510). In the case of bioinert bioceramic materials, only aphysical interdigitation of weak fibrous tissue onto the implant surfaceis obtained. If the strength of this fixation between the surroundingtissue and implant is insufficient which is often the case, thenloosening of the bioceramic can occur causing necrosis of thesurrounding tissue along with implant failure. For example, when aluminaor zirconia implants are implanted with a tight mechanical fit withinthe body and movement does not occur at the interface with tissue, theimplants can be clinically successful. However, if movement does occur,the fibrous capsule surrounding the implant can grow to become severalhundred microns thick causing the implant to loosen and leading toclinical failure.

Bioactive bioceramics include hydroxyapatite, bioglass, andbioglass-ceramics. A “bioactive” material is one that elicits a specificbiological response at its surface, which results in a beneficialbiological and chemical reaction with the surrounding tissue. Thesereactions lead to chemical and biological bonding to the tissue at theinterface between tissue and the bioactive implant, rather than mereingrowth of tissue into pores of the implant, which only providemechanical fixation. Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, JC-PDS 9-432) hasbeen of particular interest in orthopedic and dental application becausethe composition closely resembles native bone mineral and is inherentlybioactive and osteoconductive. Though hydroxyapatite has the potentialto be a load bearing implant material, applications have been limited tocoatings, porous implants and as the bioactive phase in compositesbecause most conventional calcium phosphate processing techniques havebeen unable to remove the process related defects in load bearingimplants that result in poor mechanical properties. The problemsassociated with processing hydroxyapatite materials have been solved, atleast in part, by the method disclosed in U.S. Pat. No. 6,013,591, whichdescribes the synthesis of nanometer-sized hydroxyapatite grains thatcan be densified to form a hydroxyapatite structure with improvedcompressive strength, bending strength, and fracture toughness. Theseresults can be attributed to the reduced flaw sizes inherent innanocrystalline materials.

Resorbable bioceramics include tricalcium phosphate (TCP), calciumsulfate, and other calcium phosphate salt-based bioceramics. They areused to replace damaged tissue and are eventually resorbed such thathost tissue replaces the implant. Problems long associated withresorbable bioceramics are the maintenance of strength, stability of theinterface, and matching of the resorption rate to the regeneration rateof the host tissue. Furthermore, the constituents of resorbablebiomaterials desirably are metabolically acceptable, since largequantities of material must be digested by cells. This imposes a severelimitation on these compositions. Calcium sulfate typically is used as arapidly degrading bone filler in cases where mechanical strength is notnecessary. α-TCP (α-Ca₃(PO₄)₂, JC-PDS 9-348) and β-TCP (β-Ca₃(PO₄)₂,JC-PDS 9-169) typically are used when a rapidly degrading bone fillerhaving more mechanical strength than calcium sulfate (CaSO₄, JC-PDS6-0046) is needed. Though calcium sulfate and TCP degrade rapidly, theyboth suffer from poor mechanical properties that have limited theirapplications to bone fillers.

Because calcium phosphate biomaterials are intrinsically bioactive andresorbable, they can be tailored for mechanical strength, resorption andbonding with the surrounding tissue through nanostructure. While α- andβ-TCP are widely used and while a TCP formulation having mechanical andmorphological properties advantageous for prostheses would be veryuseful, attempts to date have failed to produce reliable structural TCPimplants. Accordingly, it is an object of the invention to providetechniques for synthesizing α- and β-TCP materials, and compositesthereof, having structural and morphological properties useful forstructural implants. In particular, it is an object of the invention toprovide synthesis and processing techniques that produce a TCP materialthat can be densified under conditions that allow microstructuralcontrol, reduction or elimination of defects, ease of manufacture, andminimization of cost. It is another object of the invention to obtainTCP materials having enhanced mechanical properties, enhancedbioactivity/osteointegration and a controlled resorption profile bycontrolling the microstructure during sintering through crystal size,morphology and compositional control during synthesis and processing.

BRIEF SUMMARY OF THE INVENTION

The present invention provides compositions and articles comprisingtricalcium phosphate (TCP) materials having a particularly small crystalsize and/or particle size. The invention further provides a method ofconsolidating the TCP into a variety of articles that are either fullydense and defect free, or that possess extensive porosity.

TCP (i.e., α- and/or β-TCP) can be formed into high surface areapowders, coatings, porous bodies, and dense articles by a wet chemicalapproach. This wet chemical approach is preferred because it isversatile, simple, and easy to control, in terms of both the preparativereactions and the characteristics of the reaction product, such asmorphology, size, and reactivity. Precursor type, precursorconcentration, solvent environment, addition rate of precursors, agingtime, aging temperature, and pH during precipitation have beenidentified as the processing parameters controlling the molecular andstructural development of TCP precursor materials. Furthermore, bycontrolling dry particle formation from the precipitate through washing,drying and comminution, an ultrafine particulate TCP precursor powdercan be obtained.

This TCP precursor powder is then transformed into TCP, for example by acalcination step. The calcination temperature can be significantlyreduced with the appropriate precipitation conditions permitting theformation of an ultrafine particulate TCP that can enhance packing anddensification and lower sintering temperatures. The phase (i.e., α or β)of TCP that is obtained is dependent at least in part on theprecipitation and processing conditions and calcinations temperature andenvironment. Alternatively, a method using microwaves, X-rays, lasers,electron beams or neutron beams can be used to transform precursorpowder into TCP.

Dense TCP articles can be fabricated by pressureless orpressure-assisted sintering processes using this ultrafine TCP powder.By reducing the crystal size within an article, the smallest possibledefect size is reduced thereby increasing the highest possible strength.In addition, ceramics become more ductile at lower temperatures as thevolume fraction of grain boundaries increases allowing grain boundarysliding allowing for rapid superplastic net-shape forming. Furthermore,the resorption profile of dense TCP can be controlled by extending theheat treatment during sintering or through post-sinter thermal cycles toalter the microstructure. The subsequent controlled grain growth canthen be used to increase or decrease the resorption rate. This TCPprecursor powder is then transformed into TCP, for example by acalcination step. The calcination temperature can be significantlyreduced with the appropriate precipitation conditions permitting theformation of an ultrafine particulate TCP that can enhance packing anddensification and lower sintering temperatures. The phase (i.e., α or β)of TCP that is obtained is dependent at least in part on theprecipitation and processing conditions and calcinations temperature andenvironment. Alternatively, a method using microwaves, X-rays, lasers,electron beams or neutron beams can be used to sinter TCP, with orwithout pressure, into a dense article.

Thus, TCP of the invention possesses greater reliability and bettermechanical properties as compared to conventional TCP having a coarsermicrostructure. In addition, the TCP of the invention can bestructurally reinforced by incorporating a secondary reinforcing speciesinto the TCP precursor material during nanocomposite processing.

In one aspect, the invention provides a composition includingparticulate TCP having an average TCP crystal size of about 250 nm orless and an average particle size of about 5 μm or less. In anotherembodiment, the invention provides TCP compositions having a BET surfacearea of about 20 m²/g or greater.

In another aspect, the invention provides an article comprising aconsolidated TCP structure having an average crystal size of about 80 μmor less and a density of about 90% of the theoretical density. In yetanother aspect, the invention provides an article comprising aconsolidated TCP structure having an average crystal size of about 1 μmor less and a porosity of about 20% or greater.

The invention also provides a method of calcining a TCP precursorprecipitate at a temperature of about 400° C. to about 1400° C. andrecovering a nanostructured TCP article having a BET surface area ofabout 20 m²/g or greater and a crystal size of about 250 nm or less. Theinvention also provides a method that involves calcining a TCP precursormaterial at a temperature of about 400° C. to about 1400° C. andrecovering a nanostructured TCP article having a BET surface area ofabout 20 m²/g or greater and an average particle size of about 5 micronor less.

In another aspect the invention provides a particulate TCP compositionhaving an average crystal size small enough that the composition can besintered to a theoretical density of about 90% or greater bypressureless sintering. In another aspect, a method is providedcomprising sintering a composition comprising a TCP to a theoreticaldensity of about 90% or greater by pressure-assisted sintering. Theinvention also includes a method involving sintering TCP in the absenceof any sintering additives.

Other advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for the synthesis ofnanostructured TCP, processing of nanostructured TCP into implantablearticles, and steps for carrying out related methods. Desirably, thesemethods result in one or more improvements related to (a)microstructural control and design on the nanometer scale, (b) phaseuniformity and chemical homogeneity on the molecular level, (c)uniformity of chemical and physical properties, (d) machinability ofpartially consolidated tricalcium phosphates, (e) sintering behavior,(f) mechanical reliability and strength, (g) net shape forming, (h)manufacturing of porous and dense bodies, (i) formation of compositematerials, and/or (j) gene, drug and protein delivery devices.

The inventive synthetic methods preferably lead to exceptionalmicrostructural control over the TCP products. Accordingly, the TCPprovided in accordance with the invention preferably can be densifiedwith an ultrafine microstructure leading to reduced flaw sizes, greaterreliability, better mechanical properties (e.g., strength and fracturetoughness), improved ductility, and enhanced bioactivity compared toconventional polycrystalline TCP having a coarser microstructure.Because of the finer microstructure, TCP of the invention can bedensified without the need for sintering aids and at substantially lowertemperatures. The nanostructured TCP not only provide superiormechanical properties, but also offer the potential for superplasticnet-shape forming for inexpensive rapid prototyping.

The present invention further provides a TCP composition comprisingdeagglomerated nanometer-sized TCP particles. A wet chemical approach isused in the synthesis of preferred compositions leading to theadvantages that compositional homogeneity is provided. Furthermore, themethod is versatile and easy to control both in terms of the preparativereactions and the character of the reaction product. The processing canbe tailored for different applications such as consolidated/densifiedTCP articles, porous bodies, coatings, cements, and composites bycontrolling the morphology, size, and reactivity of the precipitatedparticles. The TCP compositions of the invention preferably comprise TCPpowder having a particle size on the order of several microns or lessand a narrow log normal particle size distribution.

Crystal size typically governs bulk properties in a consolidated ordensified article prepared from the TCP composition. Minimization of TCPcrystal size makes consolidation of the crystals, for example duringsintering, easier because smaller crystals can re-arrange and pack morereadily with respect to each other, and because agglomeration ofcrystals prior to densification is minimized which enhancesdensification. Accordingly, preferably the TCP powder of the inventionhas an average particle size that approaches the average crystal size ofthe material. In addition, the bioceramic TCP material of the inventionhaving very small crystal sizes is ideal for use in powders or coatings,and for use with bones. The crystal size of healthy bone isapproximately 20-30 nm, and bioceramic material having similar crystalsize will be more compatible with bone as a result.

Accordingly, the compositions of the invention comprise particulate TCPhaving an average crystal size of about 250 nm or less (e.g., about 220nm or less, about 200 nm or less, or about 180 nm or less). Preferably,the crystal size is about 150 nm or less (e.g., about 130 nm or less),more preferably about 100 nm or less (e.g., about 80 nm or less, orabout 50 nm or less), and most preferably about 30 nm or less (e.g.,about 20 nm or less). In some embodiments, it is desirable that theparticulate TCP have an average crystal size of about 500 nm or more(e.g., about 1 micron or more, about 3 micron or more, about 12 micronor more, or even about 60 micron or more) in order to retard the rate ofTCP resorption.

In addition, the compositions of the invention comprise particulate TCPhaving a small average particle size, in particular an average particlesize of about 5 μm or less (e.g., about 3 μm or less, about 2 μm orless, or about 1 μm or less), preferably an average particle size ofabout 800 nm or less (e.g., about 650 nm or less), more preferably anaverage particle size of about 500 nm or less (e.g., about 400 nm orless). In some embodiments, it is desirable that the particulate TCPhave an average particle size of about 100 nm or more (e.g., about 150nm or more, or about 200 nm or more). Any combination of preferredparticle size and preferred crystal size can define a preferablecombination of the invention, for example an average crystal size ofabout 150 nm or less and an average particle size of about 1 μm or less,etc. Preferably, the crystal size is determined by peak broadeninganalysis of X-ray diffraction peaks or by TEM, and particle size isdetermined by laser scatter or diffraction, or by electron microcopy(e.g., TEM or SEM).

Typically, the particulate TCP has a narrow log normal particle sizedistribution. For example, typically about 25% or more (e.g., about 50%or more, about 75% or more) of the TCP particles have a particle size ofabout 1 micron or less (e.g., about 100 nm to about 800 nm).Furthermore, 90% or more of the TCP particles have a particle size ofless than about 10 microns or less (e.g. about 7.5 microns or less,about 5 microns or less). The crystal size and particle size can bedetermined by any suitable technique, including for example thosetechniques described above.

The compositions of the invention preferably comprise TCP particleshaving a high surface area. Typically, the BET surface area is about 20m²/g or greater. Preferably, the BET surface area is about 40 m²/g orgreater (e.g., about 60 m²/g or greater, or about 80 m²/g or greater),more preferably about 100 m²/g or greater (e.g., about 120 m²/g orgreater, or about 150 m²/g or greater).

The TCP particles can have any suitable morphology, for example theparticles can have an aspect ratio of about 1:1 to about 50:1. Themorphology of the TCP particles will depend on the desired application.When the TCP particles are to be used to form a densified article,preferably the TCP composition comprises TCP particles that aresubstantially equiaxed (e.g., having an aspect ratio of about 3:1 orless, about 1.5:1 or less, or about 1:1). When the TCP particles are tobe used to form a porous consolidated article or as the reinforcingagent a dense composite article, preferably the TCP compositioncomprises TCP particles that are whisker-like (e.g., having an aspectratio of about 3:1 or more, 5:1 or more, or even 10:1 or more).

The TCP compositions of the invention desirably are prepared using a wetchemical approach. The wet chemical approach involves (i) precipating aTCP precursor material (e.g., monetite (CaHPO₄), brushite (CaHPO₄.2H₂O),hydroxyapatite, amorphous calcium phosphate, octacalcium phosphate, orcombinations thereof), (ii) recovering the TCP precursor material, (iii)milling the TCP precursor material to form a powder in which the TCPprecursor crystals are agglomerated to a minimal extent, and (iv)transforming the TCP precursor powder to form TCP. Preferably, theindividual nanocrystals of the precipitated TCP precursor materialdefine individual particles. The method optionally further comprises (v)consolidating and densifying the TCP to form a TCP material or articlehaving useful properties. A wet chemical approach is used in thesynthesis of preferred compositions leading to the advantages thatcompositional homogeneity is provided and the method is versatile andeasy to control both in terms of the preparative reactions and characterof the reaction product.

In order to produce TCP having properties tailored for a particularapplication, a series of processing parameters are provided inaccordance with the invention that affect the molecular and structuraldevelopment and chemistry of the TCP precursor material, such as agingtemperature, aging time, addition rate of reactants (such as additionrate of calcium nitrate solution to basic ammonium hydrogen phosphatesolution in TCP production), solution pH during chemical precipitation,precursor concentration and solvent environment. Parameters affectingthe agglomeration and densification of ceramic particles such as millingmethod, calcination temperature/method, and sintering temperature/methodalso are provided.

As discussed above, the wet chemical approach involves precipitating aTCP precursor material from a solution containing a calcium salt and aphosphate source. The calcium and phosphate sources can be any suitablesources, many of which are commonly known in the art. For example, thecalcium source can be selected from the group consisting of calciumnitrate and any hydrate thereof, calcium nitrite, calcium nitride,calcium acetate and any hydrate thereof, calcium hydroxide, calciumalkoxide (e.g., diethoxide, diisopropoxide, and dibutoxide), calciumcarbonate, calcium chloride, calcium chlorite, calcium hypochlorite,calcium chlorate, calcium proprionate, calcium perchlorate, andcombinations thereof. Preferably, the calcium source is calcium nitrate.The phosphate source can be selected from the group consisting ofammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammoniumphosphate, phosphoric acid, phosphorous alkoxides such astrialkylphosphates (e.g., tributylphosphate or triethyl phosphate) ortrialkylphosphites (e.g., tributylphosphite or triethyl phosphite),β-glycerophosphate, butyl acid phosphate, phosphonoacetic acid,phosphorous pentoxide and combinations thereof. Preferably, thephosphate source is ammonium phosphate, ammonium hydrogen phosphate,ammonium dihydrogen phosphate, or a combination thereof.

Desirably, the calcium salt and phosphate sources are formed as separatesolutions, stable suspensions, or emulsions that are subsequentlycombined. The solvent can be any suitable solvent. TCP precursors can beprecipitated in water, a polar organic solvent (e.g., methanol, ethanol,isopropanol, acetone, or toluene), or a mixture thereof. If water and apolar organic solvent are used as a mixture, the polar organic solventdesirably is miscible with the water. Preferably, the TCP precursorpowder is precipitated from a mixture of water, alcohol, oil andsurfactant. If water or a water and polar organic solvent mixture isused, water soluble calcium salts such as calcium nitrate and anyhydrate thereof, calcium nitride, calcium nitrite, calcium acetate andany hydrate thereof, calcium hydroxide, calcium chloride, calciumchlorite, calcium hypochlorite, calcium chlorate, calcium perchlorate,and combinations thereof and water soluble phosphate salts such asammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammoniumphosphate, phosphoric acid, β-glycerophosphate, butyl acid phosphate,phosphonoacetic acid, and combinations thereof preferably are used.

The calcium source solution and phosphate source solution can have anysuitable concentration. Typically, the concentration of the phosphatesource solution is about two-thirds the concentration of the calciumsource solution. Desirably, the calcium source solution has aconcentration of about 2 M or less (e.g., about 1.5 M or less, or about1 M or less). Optimal physical and chemical properties of theprecipitate TCP precursor material are obtained when relatively lowsource solutions are used, although yields of the TCP precursorprecipitate are reduced when using lower concentration solutions.Accordingly, calcium source solution concentrations of about 0.1 M toabout 1.5 M (e.g., about 0.12 M to about 1 M, or about 0.15 M to about0.5 M) are preferred. The phosphate source solution typically has aconcentration of about 1.3 M or less (e.g., about 1 M or less, or about0.6 M or less). Preferably, the phosphate source solution has aconcentration of about 0.05 M to about 1 M (e.g., about 0.07 M to about0.6 M, or about 0.1 M to about 0.3 M).

Preferably, the TCP precursor material is precipitated from calciumsource solutions and phosphate source solutions having a molar ratio ofcalcium to phosphorous of about 1 to about 2 (e.g., about 1.2 to about1.8). More preferably, the molar ratio of calcium source to phosphorussource is about 1.4 to about 1.6, more preferably about 1.5 (i.e., 3:2).The TCP precursor material can be formed by addition of a calcium sourcesolution to a phosphate solution, by addition of a phosphate sourcesolution to a calcium source solution, or by simultaneous mixing of acalcium salt solution and a phosphate source solution. Preferably, thecalcium salt solution is added to the phosphate source solution.

Control of the mixing rates (e.g., addition rates) of the calcium sourceto the phosphate source (or alternatively the phosphate source to thecalcium source) is advantageous for controlling the size of theresulting TCP precursor crystallites. Desirably, the addition rate ofthe calcium source to the phosphate source (or vice versa) is about 0.1mmol/min or more (e.g., about 1 mmol/min or more, about 10 mmol/min ormore, about 50 mmol/min or more, or even about 100 mmol/min or more).Preferably, the mixing rate is very large (e.g., instantaneous mixing ismost preferred); however, the actual mixing rate typically is limited bythe mixing/agitation equipment being used and generally is about 1mol/min or less (e.g., about 0.8 mol/min or less, or about 0.6 mol/minor less). Preferably, the mixing rate (e.g., addition rate) is about 1mmol/min to about 1000 mmol/min, more preferably about 10 mmol/min toabout 500 mmol/min.

The pH of the calcium and phosphate solutions has been found to be animportant parameter for controlling the type of TCP precursor materialthat is formed. Desirably, the TCP precursor material is precipitatedfrom a solution having a pH of from about 5 to about 11, more preferablyfrom about 7 to about 10. When the solution pH is about 5 or 6, the TCPprecursor material typically comprises monetite, brushite, or acombination thereof. When the solution pH is 10 or above, the TCPprecursor material typically comprises a poorly crystalline apatiticcalcium phosphate material. When the solution pH is about 7 to about 10,the TCP precursor material typically comprises predominantly amorphouscalcium phosphate, octacalcium phosphate (Ca₈H₂(PO₄)₆.5H₂O), apatiticTCP, or a combination thereof. The pH of the precursor solutions can beadjusted by addition of one or more common pH adjustors. The pH adjustorcan be any suitable pH adjustor, for example nitric acid, acetic acid,ammonium hydroxide, or tetramethylammonium hydroxide (e.g.tetraethylammonium hydroxide or tetrabutylammonium hydroxide).Preferably, the pH adjustor is nitric acid, ammonium hydroxide, or acombination thereof.

The precipitated TCP precursor material is then recovered from thereaction mixture, for example by filtering, filter pressing,centrifugation, or settling and decantation. Preferably, the TCPprecursor material is aged prior to recovery. The TCP precursor materialcan be aged at any suitable temperature and for any suitable amount oftime. Typically, the TCP precursor material is aged at a temperaturebetween about 0° C. and about 90° C., preferably between about 5° C. andabout 50° C., and more preferably between about 10° C. and about 30° C.(e.g., about 20° C.). Typically, the TCP precursor material is aged forabout 1 minute or more (e.g., about 30 minutes or more, or about 60minutes or more). Preferably, the TCP precursor material is aged forabout 2 hours or more (e.g., about 5 hours or more, about 10 hours ormore, about 30 hours or more, about 50 hours or more, or even about 100hours or more). After aging, the TCP precursor material can be collectedand then redispersed in a solution having the same solvent and pH as thereaction solution.

The recovered TCP precursor material desirably is dried to form a powderand then is milled. The dry TCP precursor powder can be milled by anysuitable method and in the absence or presence of any suitable solvent.Preferably, the dry TCP precursor powder is milled in the presence ofanhydrous alcohol, acetone, toluene, or a combination thereof. Aftermilling, the dry TCP precursor powder is dried again.

The dried and milled TCP precursor powder is then transformed into a TCPpowder, preferably a nanocrystalline TCP powder. Typically, the TCPprecursor powder is calcined under a set of conditions that allowdehydroxylation and production of a robust TCP material having theproperties described above. Desirably, calcination of the TCP precursormaterial produces a pure phase α-TCP or β-TCP, although many otherproducts can be formed. Such other products can include mixed-phasematerials, for example hydroxyapatite/α-TCP, hydroxyapatite/β-TCP,α-TCP/β-TCP, and hydroxyapatite/α-TCP/β-TCP. The composition andproperties of the TCP material formed by calcination will depend, atleast in part, on the calcination conditions, such as the temperature,temperature ramp rate, time, cooling rate, and oven atmosphere.Desirably, the calcination temperature is about 400° C. to about 1400°C. (e.g., about 500° C. to about 1300° C., or about 600° C. to about1200° C.). Pure phase β-TCP typically is formed by calcining in vacuumat a temperature of about 400° C. to about 900° C. (e.g., about 600° C.to about 800° C.). Pure phase α-TCP typically is formed by calcining ata higher temperature of about 1000° C. to about 1400° C. (e.g., about1100° C. to about 1250° C.). A mixed α-TCP/β-TCP can be formed bysoaking the TCP precursor powder at a first calcination temperature thatis greater than 1000° C. (i.e., to allow for full or partial formationof α-TCP) and then soaking the TCP precursor powder at a secondtemperature that is less than 1000° C. (i.e., to allow for partialformation of β-TCP). Of course, a mixed α-TCP/β-TCP material also can beproduced by soaking the TCP precursor powder at a first temperature thatis less than 1000° C. (i.e., to allow for full or partial formation ofβ-TCP) and then soaking the TCP precursor powder at a second temperaturethat is greater than 1000° C. (i.e., to allow for partial formation ofα-TCP).

The ramp rate will depend in part on the type of calcination apparatusthat is being used and the type of material being calcined. The ramprate typically is very rapid and is limited only by the ability of theoven being used to produce a linear well-controlled heating temperature.When the TCP precursor material further comprises an organic materialthat is to be removed by heating, the ramp rate can be slower to ensurecomplete removal of that organic material.

The calcination time typically is about 15 min or more (e.g., about 30min or more, or about 1 hour or more) and about 15 hours or less (e.g.,about 12 hours or less, or about 10 hours or less). Preferably, thecalcination time is about 1 hour to about 4 hours, more preferably about1.5 hours to about 2.5 hours (e.g., about 2 hours). Generally, shortcalcination times (e.g., about 1 hour or less) produce composite TCPmaterials having smaller crystal sizes, while longer calcination times(e.g., about 4 hours or more) produce pure phase TCP having largercrystal sizes. A calcination time of about 1 hour to about 4 hourstypically produces pure phase nanocrystalline TCP powder. The coolingrate (i.e., quenching rate) following calcination will depend on thetype of material being produced. For example, when producing α-TCP thecooling rate desirably is rapid to avoid formation of β-TCP.Contrastingly, when producing β-TCP the cooling rate is less important.

The TCP precursor material desirably is calcined in the presence of areducing atmosphere. The reducing atmosphere can be any suitablereducing atmosphere; for example, the atmosphere can be a vacuum or agaseous atmosphere comprising nitrogen, argon, helium, hydrogen, andmixtures thereof. Preferably, the reducing atmosphere is a vacuum or agaseous atmosphere comprising nitrogen.

As an alternative to the thermal treatments (e.g., calcination)typically used to form α- or β-TCP from the TCP precursor material,X-rays, microwaves, electron beam, or other similar radiation can beused to form TCP from the TCP precursor material and still maintainsmall crystal sizes. By controlling the intensity of the radiation beamand length of exposure, the desired TCP phase can be formed. Forexample, β-TCP can be transformed into α-TCP having an average crystalsize of about 50 nm by exposure to a high intensity and high energyX-ray beam for about 5 minutes.

Using TCP synthesis via the wet chemistry route provided in theinvention, a variety of useful applications are realized. First, TCPpowders are provided which can be used as bone grafts, bone substitutes,void fillers, pastes, or cements. Second, TCP powders can be used toform TCP coatings including, for example, thermal spray coatings,liquid-based coatings, sputtered coatings, vapor-phase coatings,coatings via wet chemical methods, and the like, many of which are knownin the art. Such coatings can benefit from the composition of theinvention as the very small particle size results in higher-quality andbetter-adherent coatings. Porous coatings can be made by admixing anorganic species with the bioceramic, forming the coating, and burningout the organic material. Similarly, self-assembled surfactants can beused to form very small pores. For larger pore articles, a polymer canbe admixed with the bioceramic crystalline powder and burned out aftersolidification. Third, the TCP compositions of the invention are easilyformable by net shape forming, green machining, or machining aftersintering because of their small crystal and particle size.

In one aspect of the invention, the TCP compositions are provided asconsolidated particulate TCP, where “consolidated” is meant to define acollection of TCP particles that forms a self-supporting structure. TCPcan be consolidated by any suitable technique, for example by providingparticulate TCP in a press and compressing the TCP to form an article.The consolidated particulate TCP can be dense or porous. It hasgenerally been relatively straightforward to make porous ceramicarticles, but significantly more difficult to make dense ceramicarticles. The very small TCP particle size of the invention allowsformation of very dense articles. Such dense, strong materials can beused as implants, in particular as load-bearing implants (e.g., dentaland orthopedic implants) where strength is required, such as pins,screws, threaded bodies, inter-body spacers, and plates for fracturefixation and fusion, spinal fusion, ball joints for hips, crowns forteeth, etc. The consolidated article also can be formed into the shapeof a prosthesis, or can define at least part of a prosthesis such as anexterior coating on a prosthesis. In a particularly preferredembodiment, the consolidated and densified TCP article is used as aspinal implant, an internal or external fixation implant, or an implantfor soft tissue attachment, the shapes and dimensions of which arecommonly known in the art. Spinal implants can be in the form of a screwand plate, a vertebral body replacement, or an inter-body spacer. Inother preferred embodiments, a densified TCP article of the inventioncan be modified so as to have a bored hole that is filled with asecondary additive such as a polymeric additive (e.g., a polymer sponge,or collagen) which optionally contains one or more biological orpharmaceutical additives as described above.

The consolidated TCP article can have any suitable dimensions. Thedimensions will depend on how the consolidated TCP article is beingused, for example, the type of implant, prosthesis, orimplant/prosthesis coating. The dimensions for such articles arecommonly known in the art. Typically, the consolidated TCP article willhave a minimum dimension of about 0.5 cm or greater (e.g., 0.8 cm orgreater, about 1 cm or greater, or about 2 cm or greater). For example,when used as an exterior coating on a prosthesis, the consolidated TCParticle is about 0.5 μm thick or greater (e.g., about 1 μm or greater)in at least one region, and has a lateral dimension of about 0.5 cm orgreater (e.g., about 1 cm or greater) relative to the article coated. Insome embodiments, the consolidated TCP article has a maximum dimensionof about 10 cm or less (e.g., about 7.5 cm or less, or about 5 cm orless).

Typically, the consolidated article has an average crystal size (e.g.,grain size) of about 80 μm or less (e.g., about 75 μm or less, about 50μm or less, or about 25 μm or less). Preferably, the consolidatedarticle has an average crystal size of about 10 μm or less (e.g., about1 μm or less, about 750 nm or less, about 500 nm or less, or about 300nm or less). In some embodiments, it is desirable that the consolidatedarticle has an average crystal size of about 100 nm or more (e.g., about150 nm or more, or about 200 nm or more). The consolidated articlepreferably has a crystal size distribution of about ±0.75 (e.g., about±0.5, about ±0.25, or about ±0.1) times the average crystal size.

The theoretical density of consolidated articles of the inventionpreferably is about 25% or greater, more preferably about 40% orgreater, and even more preferably about 55% or greater. In a preferredembodiment, the TCP powder is formed into a densified particulate TCParticle where “densified” is defined as having undergone a densificationstep to create a self-supporting article. Preferably, the TCP powder isdensified to a theoretical density of about 60% or greater (e.g., about70% or greater, or about 80% or greater). More preferably, the articlehas a density that is about 90% or more (e.g., about 95% or more, orabout 98% or more) of the theoretical density.

The densified articles typically have a compressive strength (ASTM C1424-99) of about 150 MPa or greater (e.g., about 300 MPa or greater),preferably about 500 MPa or greater (e.g., about 600 MPa or greater, orabout 700 MPa or greater). The three-point bending strength (ASTMC1161-94) typically is about 100 MPa or greater (e.g., about 200 MPa orgreater), preferably about 300 MPa or greater (e.g., about 400 MPa orgreater). Generally, the three-point bending strength is about 700 MPaor less (e.g., about 600 MPa or less). The densified articles typicallyhave a fracture toughness (ASTM C 1421-01a) of about 0.5 MPa·m^(1/2) orgreater (e.g., about 1 MPa·m^(1/2) or greater, or about 1.5 MPa·m^(1/2)or greater). Generally, the fracture toughness is about 5 MPa·m^(1/2) orless (e.g., about 4 MPa·m^(1/2) or less). Such densified TCP articlescan be partially or fully transparent. Preferably, the articles are ableto transmit about 50% or more (e.g., about 70% or more, or about 90% ormore) light having a wavelength in the range of about 150 nm to about1,000 nm.

The ability to readily densify the TCP material of the inventionindicates that the TCP material also is of a quality that can make itvery useful for applications that do not necessarily require density.That is, densification can be used as a screening test for aparticularly useful composition, and many compositions of the inventionare referred to as densifiable under certain conditions but need notnecessarily be densified. As such, the TCP compositions of the inventioncan also be used to make relatively porous materials/articles for use inapplications requiring high-surface-area, flowable, castable materialssuch as cement for teeth, cement for cranial surgery, and the like. Insome cases, porosity can be tailored for a particular purpose such asfor bone ingrowth where pores of approximately 200 μm may be desirable.

The porosity of these materials/articles desirably is about 20% orgreater. Preferably, the porosity is about 30% or greater (e.g., about40% or greater, or about 50% or greater). More preferably, the porosityis about 60% or greater (e.g., about 70% or greater). The average poresize typically is about 300 μm or less (e.g., about 200 μm or less,about 150 μm or less, about 100 μm or less). Preferably, the averagepore size is about 50 μm or less (e.g., about 20 μm or less, or about 10μm or less). Typically the average pore size is about 25 nm or more(e.g., about 50 nm or greater, about 100 nm or greater). Preferably, theaverage pore size is about 200 nm or greater (e.g., about 500 nm orgreater, or about 1 μm or greater).

The consolidated porous articles can have a compressive strength (ASTMC1424-99) of about 50 MPa or greater (e.g., about 100 MPa or greater, orabout 150 MPa or greater). In addition, the consolidated porous articlescan have a three-point bending strength (ASTM C1161-94) of about 20 MPaor greater (e.g., about 40 MPa or greater, or about 60 MPa or greater).Generally, the compressive strength is about 500 MPa or less and thethree-point bending strength is about 400 MPa or less. The consolidatedporous articles typically have a fracture toughness (ASTM C1421-01a) ofabout 0.2 MPa·m^(1/2) or greater (e.g., about 0.5 MPa·m^(1/2) orgreater). Generally, the fracture toughness is about 1 MPa·m^(1/2) orless.

In other embodiments, the densified article comprising TCP has a voidvolume of about 75% or less (e.g., about 50% or less, about 25% or lessabout 10% or less or about 5% or less). Such a densified article neednot entirely consist of TCP, rather the article can be a biphasic orcomposite TCP article. For example, the article can consist of a porousTCP structure, wherein the porosity is filled in by the presence of asecondary additive such as a structural additive (e.g., hydroxyapatite,silver, gold, or magnesium alloy) or an organic additive (e.g., apolymer). Such composite materials will be described in further detailherein.

Typically, the consolidated TCP article described above is prepared froma TCP powder (e.g., a calcined TCP powder) by sintering under mildconditions. The consolidated TCP structures can comprise β-TCP, α-TCP,or a mixture thereof. Typically, the calcined TCP powder is compactedand fired at a sintering temperature of about 400° C. to about 1400° C.(e.g., about 600° C. to about 1300° C.). The sintering time typically isabout 30 minutes or more (e.g., about 60 minutes or more) and about 3hours or less (e.g., about 2 hours or less). To form a sintered β-TCP,compacted β-TCP powders are sintered at a temperature of about 1150° C.or less. To form a sintered α-TCP, compacted α-TCP powders are sinteredat a temperature of about 1150° C. or more. To form a α-TCP/β-TCPsintered composite, compacted α-TCP powder can sintered at a temperatureof about 1150° C. or less, compacted β-TCP powders can be sintered at atemperature of about 1150° C. or more, or a compacted mixture of α-TCPand β-TCP powders can be sintered at a temperature of about 400° C. toabout 1400° C. (e.g., about 600° C. to about 1200° C.).

The calcined compositions of the invention can be sintered to a hightheoretical density as discussed above without the need for “sinteringaids,” many of which are known in the art, including glasses and lowmelting point glassy oxides that become highly viscous and flow freelyduring sintering but results in an interfacial glassy phase that weakensan article formed therefrom.

In one preferred embodiment, the TCP compositions of the invention aredensified without external pressure (i.e., via pressureless sintering).Pressureless sintering generally is carried out at a low sinteringtemperature and for relatively short periods of time. For example, thesintering time typically is about 2 hours or less, preferably about 1hour or less, more preferably about 30 minutes or less. Pressurelesssintering can be used because of the unique nature of the TCP materialof the invention. In particular, the average particle size and particlesize distribution of the TCP powder of the invention desirably is suchthat the composition can be pressurelessly sintered to a theoreticaldensity of about 90% or more, preferably about 95% or more, and morepreferably about 98% or more at a sintering temperature of about 400° C.to about 1400° C. (e.g., about 600° C. to about 1200° C.).

In another preferred embodiment, the consolidated and/or densified TCParticles of the invention can be formed by colloidal pressing (i.e., wetpressing), hot pressing, or hot isotactic pressing. Colloidal pressingis a process by which a stabilized sol of TCP precursor material,optionally containing binders or lubricants, is uniaxially pressed in adie to remove the solvent. A stabilized sol of material is defined as asuspension of particles, which do not undergo sedimentation appreciablyover time. Frits within the die allow the solvent to escape as the dieis pressurized while trapping the solid particles. Once enough solventis removed to obtain a solid pellet, the pellet is removed and iscarefully dried to prevent drying stresses from cracking the pellet.After fully drying the pellet, the pellet is cold isostatically pressed(CIPed) and then undergoes pressureless sintering as discussed above.Colloidal pressing prevents particle agglomeration that is oftenassociated with working with a dry powder, and benefits from thelubrication effects of the solvent during pressing, which allow theparticles in solution to rearrange into the densest packing.

Hot pressing is a form of pressure-assisted sintering whereby a pressureis applied uniaxially to a powder contained within the die duringsintering to obtain a fully dense plate. This plate can then be machinedinto the desired shape. Hot isostatic pressing is a form ofpressure-assisted sintering whereby a pressure is applied isostaticallyto a formed part. The part can be sintered to closed porosity or can bean encapsulated green body that has prepared by well-known net shapeforming techniques such as cold isostatic pressing, green machining slipor gel casting, or injection molding. The pressure-assisted sinteringallows for more rapid densification and a lower sintering temperature.Typically, a pressure of about 10 MPa or more and about 1 Gpa or less(e.g., about 500 MPa or less, or about 250 MPa or les) and a sinteringtemperature of about 400 to about 1200° C. is used in pressure-assistedsintering. Generally, the use of higher sintering pressure enables theuse of a lower sintering temperature.

In all of the compositions, articles, and methods described above, thepreferred compositions, articles, and products of methods comprise TCPeither alone or optionally in combination with a secondary additive todefine a composite article. The secondary additive can be a structural,organic, polymeric, biological and/or pharmaceutical additive. Thesecondary additive can be present in any suitable amount and preferablyis present in an amount ranging from about 1% to about 50% (e.g., about5% to about 40%) by volume, preferably from about 15% to about 35% byvolume. In a preferred embodiment, the secondary additive and TCPmaterial are each nanocrystalline so as to form a “nano/nano” compositematerial.

Composites provided in the invention, in particular zirconia-toughenedTCP, possess even better mechanical strength than pure TCP and have thepotential as material of choice for load-bearing applications. Thechemical precipitation process of the invention can also be modified toprovide a variety of other novel products such as coatings, cements,pastes and drug/gene delivery.

Composites of TCP with a secondary additive can be formed by anysuitable method. For example, the TCP precursor material can beprecipitated from a solvent as described above, wherein the solventcontains, in suspension, one or more secondary additives, or the TCPprecursor material can be provided in suspension in a solvent from whichis precipitated the secondary additive. Preferably, the TCP precursormaterial and secondary additive(s) are co-precipitated essentiallysimultaneously. Alternatively, the TCP precursor material can becalcined or sintered in the presence of the secondary additive. In yetanother method, the TCP powder can be independently recovered and thesecondary additive independently provided (rather than precipitationfrom a common solvent or suspension), and subsequently admixed andsintered.

Structural additives can be added to the TCP to structurally reinforcethe nanocomposite material. The structural additive can be any suitablestructural additive. Suitable structural additives include ceramics,metals, alloys, and combinations thereof. Ceramics preferred for use incomposites include metal oxides (e.g., alumina, zirconia, and titania),silicon carbides, silicon nitrides, combinations thereof, and otherstructural ceramics. Metals preferred for use in composites include Mg,Ti, Ta, Nb, Al, Ni, W, Fe, Mo, Co, Zr, Au, Ag, V, alloys thereof,stainless steel, combinations thereof, and other structural metals.Other suitable structural additives include apatite and carbon. Thestructural additive can have any suitable size or shape. For example,the structural additive can have the shape of particles, rods, whiskers,plates, nanotubes, or fibers. In particular, structural additives havingnon-spherical aspect ratios are desirable and contribute to greatimprovements in the fracture toughness and strength. Preferably, thestructural additive is selected from the group consisting ofnanocrystalline alumina plates, hydroxyapatite whiskers, carbon fibersor nanotubes, silver particles or rods, zirconia particles or rods, andcombinations thereof. The structural additive should be selected tostrengthen the composite. The secondary, non-TCP structural componentcan form a major or minor component, with the overall composite havingat least 10% TCP, preferably at least 20% TCP, more preferably at least50% TCP.

Zirconia and alumina are used advantageously in compositions whentoughening of a composition is desired. Compositions can be formulatedbased on mechanical properties desired. For example, if a secondaryphase is “pinned” at grain boundaries (e.g., forms an intergranularphase), ultra-fine crystal sizes can be maintained by preventing graingrowth of the major phase, which strengthens the material by reducingthe defect size. The secondary phases can also deflect or bridge cracksand transformation toughen absorbing crack energy, thereby strengtheningthe material.

The organic additive can be any suitable organic additive, for example asurfactant (e.g., a cationic surfactant such as cetyl triammoniumbromide or dodecyltrimethylammonium chloride; anionic surfactants suchas sodium stearate, calcium stearate, zinc stearate, sodiumdisopropylnaphtalene sulfonate or other alkali or ammonium citrates,acrylates, sulfonates, sulfates, lignosulfonates, carboxylates andphosphates; and nonionic surfactants such as ethoxylated nonylphenol,ethoxylated tridecyl alcohol, acetylenic diol). The polymeric additivecan be any suitable polymeric additive, for example a polymer selectedfrom the group consisting of polylactic acid, polyglycolic acid,polylactic/polyglycolic acid copolymers, polypropylenefumarate,polyhydroxybutyric acid, polyhydroxyvaleric acid, polycaprolactone,polyhydroxycarboxylic acids, polybutyrene succinate, polybutyleneadipate, collagen, chitosan, alginate, cellulose, starches, sugars,polypeptides, polyethylene glycol, vinyl pyrrolidones, acrylamides andmethacrylates or any of their derivates, or a copolymer micelle such asthe triblock copolymer PEO-PPO-PEO, PPO-PEO-PPO,polyvinylpyridine-polystyrene-polyvinylpyridine (PVP-PS-PVP), PS-PVP-PS,PS-PEO-PS, PEO-PS-PEO, etc. The biological additive can be any suitablebiological additive, for example plasmid DNA or RNA or proteins (e.g.,bone morphogenetic proteins 2, 4, 7). The pharmaceutical additive can beany suitable pharmaceutical additive, for example bisphosphonates (e.g.,alendronate) and cis-platinum, antibiotics, anti-inflammatories,anti-arthritism, erythropoietin, etc.

In one preferred embodiment, the TCP porous articles described above areinfiltrated with a secondary additive such as hydroxyapatite to form afully dense article. This composite article will have sufficientstrength for load-bearing applications. After implantation, the TCP willbe substantially resorbed leaving a porous structure of the secondarycomposition (e.g., hydroxyapatite) into which bone will ingrow. Inanother preferred embodiment, a consolidated article (e.g., an implant)comprising a TCP precursor material such as hydroxyapatite is convertedto a TCP composite article (e.g., a biphasic hydroxyapatite/TCPcomposite article). The TCP precursor material can be converted by anysuitable means. Preferably, the TCP precursor material is convertedthrough the use of a laser light source (e.g., x-ray, UV, electron, orneutron beam) as described above. For example, the surface of aconsolidated or densified hydroxyapatite article can be converted toα-TCP and/or β-TCP. Using a laser beam is particularly advantageousbecause the laser can convert the TCP precursor material in predictableways (e.g., in selected areas of an implant). A biphasichydroxyapatite/TCP article will have the strength and structuralstability of hydroxyapatite combined with the resorptive properties ofTCP. The amount of TCP formed on the surface of the article will dependon the penetration of the laser into the surface and the length of timefor exposure. Typically, the time of exposure is about 1 min to about 20min (e.g., about 2 min to about 10 min, or about 3 min to about 7 min).Desirably, about 1 μm to about 250 μm (e.g., about 5 μm to about 125 μm)of the hydroxyapatite surface is converted to TCP, which is more readilyresorbed than hydroxyapatite.

The articles and compositions of the invention desirably have aresorption time of about 1 month or more (e.g., about 3 months or more,about 6 months or more, or about 1 year or more). The rate of resorptionwill depend at least in part on the crystal size of the composition orarticle. Smaller crystal sizes will be resorbed more rapidly than largercrystal sizes. The desired resorption rate will depend on theapplication and the crystal size can be tailored to match a desiredresorption rate. In some applications, it is desired that the resorptiontime be about 6 months or more (e.g., about 1 year or more, or about 2years or more).

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the examples below. Thefollowing examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

Example 1 Optimization of Nanocrystalline TCP Synthesis for Sintering

The sinterability of nanocrystalline TCP powders can be improved byoptimizing synthesis parameters such as precursor concentration,addition rate, pH, aging time and aging temperature to produce ananocrystalline TCP powder that will sinter to high density (e.g.greater than about 95% theoretical density) while a maintaining ananocrystalline microstructure by pressureless sintering.

Precursor solutions containing either 7.5 liters aqueous (NH₄)₂HPO₄solution (NHP) or 7.5 liters aqueous Ca(NO₃)₂ solution (CaN) areprepared at various concentrations. The precursor solutions containenough NHP and CaN to maintain a calcium to phosphate ratio of about3:2. The pH of the calcium and phosphate precursor solutions is adjustedfrom about 5 to about 11 by adding either concentrated nitric acid or anorganic acid such as acetic acid to lower the pH or adding eitherconcentrated ammonium hydroxide or an organic base such as tetramethylammonium hydroxide. The precursor solutions are mixed at flow ratesranging from about 20 ml/min to about 240 ml/min using a high-speed andhigh-shear mixer to achieve near-instantaneous high energy mixing. Theprecursor solutions can be mixed (e.g., combined) in any order. Forexample, the CaN solution can be added to the NHP solution, or the NHPsolution can be added to the CaN solution for a batch or semi-batchprocess. Alternatively, the CaN solution and the NHP solution can besimultaneously added to the mixer for a semi-batch or continuousprocess.

Once the addition has been completed, the combined solutions are stirredand aged at temperatures ranging from about 0° C. to about 90° C. forabout 0 to about 100 hours. After aging, the precipitate is collected bycentrifugation, filtering or settling and the supernatant is decanted.The precipitate is then redispersed in a solution having the same pH asthe decanted supernatant. This washing procedure is repeated two moretimes. Subsequently, the precipitate is redispersed and washed withanhydrous alcohol (i.e., methanol, ethanol, isopropanol, etc.), acetoneor toluene three more times to de-water the precipitate. The nowgelatinous precipitate is then dried. The dried TCP precursor powder isthen milled in anhydrous alcohol, acetone or toluene and then driedagain.

The milled powders are then calcined in nitrogen during the ramp andunder vacuum at the soak temperature of about 650° C. for about 2 hoursto fully transform the TCP precursor powder into β-TCP. Aftercalcination, the β-TCP powders are uniaxially pressed in stainless steeldies to a pressure of about 150 MPa. These compacted pellets are thencold isostatically pressed (CIPed) at a pressure of about 300 MPa forabout 3 minutes. After CIPing, the pellets are then sintered in oxygenby pressureless sintering to a soak temperature of about 1100° C. forabout 2 hours to evaluate the sinterability of the calcined TCP powdersin terms of density and microstructure.

Example 2 Determination of Optimal Conditions—Effect of PrecursorConcentration

By varying the precursor concentration, the kinetics of TCP precursorsynthesis can be affected. By increasing the precursor concentration,the solubility limit at a given pH is more rapidly exceeded, creating aburst of primary nuclei for crystal growth. However, as the reactantsare continually added, the primary nuclei continue to grow rapidly.Consequently, high precursor concentrations resulted in largercrystallite and particle sizes.

In this example, TCP precursor powders can be synthesized with CaN andNHP concentrations as high as 1.5 M and 1.0 M, respectively, at anaddition rate of about 250 ml/min, at a temperature of 25° C., at anaging time of 100 hours and at a pH of about 8.5. These conditions willresult in a TCP with crystallite sizes greater than about 80 nm,particle sizes about 8 μm, and surface areas less than about 50 m²/g. Byreducing the precursor concentration, the primary nuclei grow moreslowly. For example, when TCP precursor powders are precipitated fromsolutions having CaN and NHP concentrations as low as 0.15 M and 0.1 M,respectively, at an addition rate of about 250 ml/min, at a temperatureof 25° C., at an aging time of 100 hours, and at a pH of about 8.5,crystallite sizes less than about 30 nm, particles sizes less than about1 μm and surface areas greater than about 125 m²/g will be achieved.Finally, lower precursor concentrations are preferred because theseconditions will result in theoretical densities exceeding 95% afterpressureless sintering. When TCP precursor powders synthesized at highprecursor concentration are pressurelessly sintered, theoreticaldensities only about 90% will be achieved. However, high precursorconcentration are preferred if the TCP materials produced by thesereactions are to be used as a coating, porous bodies, cements, pastes,or void fillers.

Example 3 Determination of Optimal Conditions—Effect of Addition Rate

By varying the precursor addition rate, nucleation and crystal growthrates can be controlled. Rapid addition of precursors results inlocalized high concentrations of precursors, exceeding the solubility ofTCP in those regions, which favors nucleation and formation of smallcrystals. At the maximum flow rate of about 250 ml/min at CaN and NHPconcentrations of 0.15 M and 0.1 M, respectively, at a temperature of25° C., at an aging time of 100 hours and at a pH of about 8.5, TCPcrystallite sizes of less than 30 nm, an average particle size about 1μm and surface areas greater than about 125 m²/g after calcination canbe prepared. Furthermore, the use of a high-speed, high-shear,high-energy mixer prevents the formation of non-uniform particlemorphology and distribution and ensures chemical homogeneity. Slowaddition of precursors results in a regime favoring crystal growth andformation of larger particles. At the minimum flow rate of about 20ml/min at CaN and NHP concentrations of 0.15 M and 0.1 M, respectively,at a temperature of 25° C., at an aging time of 100 hours and at a pH ofabout 8.5, crystallite sizes greater than about 80 nm, an averageparticle size of 3 μm and a surface area less than about 100 m²/g aftercalcination can be prepared. When sintered, the higher addition rateswill result in a theoretical density greater than 95% whereas the loweraddition rates will result in a theoretical density only greater thanabout 90%.

Example 4 Determination of Optimal Conditions—Effect of pH

Two parameters govern which phase will form for a given calciumphosphate: 1) the initial calcium to phosphate ratio of the reactantsand 2) the pH at which the reaction occurs. In this example, allreactions are conducted with a calcium to phosphorus ratio of about 1.5to favor the synthesis of the TCP precursor phase at CaN and NHPconcentrations of 0.15 M and 0.1 M, respectively, at an addition rate ofabout 250 ml/min, at a temperature of 25° C., and at an aging time of100 hours. At an initial pH of about 6, the as-precipitated precursorphase is a monetite/brushite phase. At an initial pH of about 7, theas-precipitated precursor phase is also a monetite/brushite phase. At aninitial pH of about 10, the as-precipitated precursor phase is a poorlycrystalline apatite. When TCP precursor materials precipitated at a pHabout 7 or lower are calcined, the monetite and brushite phases persistand β-TCP does not form. When TCP precursor materials precipitated at apH about 10 or greater are calcined, the apatitic phase persists.However, β-TCP is formed when the calcination temperature is increasedabove about 800° C. Use of reaction pHs greater than about 10 isundesirable because hydroxyapatite is the thermodynamically favoredphase at those pHs, regardless of the initial Ca/P ratio, requiring theuse of higher calcination temperatures to form TCP. At pHs about 7 orbelow, TCP is soluble and acidic calcium phosphates are favored. Theseexperiments indicate that the preferred pH range is above about 7 andbelow about 10 for obtaining a TCP precursor that can be calcined at650° C. to form a β-TCP. TCP powders prepared within this pH range willpossess a crystal size of about 50 nm or less, a surface area of about150 m²/g or more and a narrow particle size distribution with an averagesize of 1 μm. When sintered under pressureless conditions, a theoreticaldensity of greater than 95% can be achieved.

Example 5 Determination of Optimal Conditions—Effect of Aging Time

The crystallinity and structural development of TCP is also affected byvarying the aging time of the precipitate. By increasing the aging time,the TCP precursor precipitate undergoes recrystallization via Ostwaldripening. As a result, occluded impurities are removed and crystalstrain is reduced as free energy of the crystal decreases, while thecrystal structure becomes perfected and the exposed area is decreased.Non-uniform morphologies such as needles, rods, or whiskers redissolveand are recrystallized in more orderly morphologies such as spheres(e.g., having an aspect ratio of about 3:1 or less) with the shapes ofthe primary particles approaching a homogeneous distribution.Furthermore, longer aging times also ensure that the reagents are fullyreacted and precipitate out of the solution. In this example, allreactions occur at CaN and NHP concentrations of 0.15 M and 0.1 M,respectively, at an addition rate of about 250 ml/min, at a temperatureof 25° C. and at a pH of about 8.5.

For applications requiring sintering, aging times approaching 100 hoursare preferred since TCP possessing a theoretical density greater than95% with a crystal size less than about 250 nm can be obtained. The TCPpowders after aging at about 100 hours will possess a crystal size lessthan about 50 nm, a surface area greater than about 150 m²/g and anarrow particle size distribution with an average particle size of lessthan about 1 μm. TCP powders aged at shorter aging times, such as 12hours, 24 hours, or 48 hours, will possess similar crystal sizes,surface areas and particle size distribution. However, these aging timeswill not achieve theoretical densities greater than about 95% whensintered because of the non-uniform morphologies.

Example 6 Determination of Optimal Conditions—Effect of AgingTemperature

By altering the aging temperature, the crystal nucleation and growth canbe controlled. By precipitating at low temperatures below about 30° C.,crystal growth can be minimized resulting in finer crystals; however,these materials typically possess poor structural development andpossess a chemically and thermally unstable structure. When aged attemperatures above about 30° C., the precipitates undergo greatercrystal growth and possess better structural development and chemicaland thermal stability. In this example, all reactions occur at CaN andNHP concentrations of 0.15 M and 0.1 M, respectively, at an additionrate of about 250 ml/min, at an aging time of 100 hours and at a pH ofabout 8.5.

Since the as-precipitated phase is a precursor that requires calcinationor a related technique to obtain TCP, conditions favoring a thermallyunstable precursor that can be easily transformed into TCP arepreferred. In particular, aging temperatures below 30° C. are preferred.Furthermore, lower aging temperatures are preferred for sintering sincethe diffusivities of these materials are higher than for materials thathave undergone higher aging temperatures. However, the solubility ofcalcium phosphate precipitates increase as temperature decreases.Consequently, the calcium to phosphate ratio in the reactant solutionsdesirably will correspondingly increase to compensate for the increasedsolubility. For example, when the calcium to phosphate ratio of thereactant solutions are set to about 1.67, a TCP precursor precipitatewith a calcium to phosphate ratio of 1.5 will be obtained when aging at0° C. After calcination, this TCP powder will possess a crystal with anon-uniform morphology and crystal size greater than about 80 nm, aparticle size greater than 3 μm and a surface area greater than about 50m²/g. When this precipitate is calcined and pressurelessly sintered, aTCP possessing a theoretical density greater than 95% will be obtainedwith a crystal size greater than about 1 μm. As a result, an agingtemperature of below 30° C. is preferred when the stoichiometry of theprecursor solutions is adjusted to compensate for the higher solubilityat lower temperatures.

When the TCP precursor is aged at about 30° C., the powder propertieswill be more refined. The resulting TCP will possess uniform crystalmorphology, a crystal size of less than 50 nm, a narrow particle sizedistribution, an average particle size of less than about 1 μm, and asurface area of greater than 150 m²/g. These powders also can bepressurelessly sintered to 95% of theoretical density with a crystalsize of less than about 250 nm. When the TCP precursor is aged at about75° C., the TCP crystals will become increasingly anisotropic. This TCPwill possess elongated crystal morphology and size greater than 150 nm,an average particle size of greater than about 5 μm, and surface areagreater less than about 50 m²/g. These powders could not bepressurelessly sintered to 95% of theoretical density.

SUMMARY OF EXAMPLES 1-6

Nanocrystalline TCP can be synthesized by chemical precipitationfollowed by calcination. The effects of precursor concentration, pH,addition rate, aging time, aging temperature, and calcinationtemperature on the crystallite size, stoichiometry, particle size anddistribution, morphology, crystallinity and structural development canbe examined. By identifying the important processing parameters and themethod by which they can be controlled, the crystallite size andprocess-related defect structures can be reduced to enhance themechanical properties of bulk TCP. Furthermore, using the parameters toreduce agglomeration, to control the particle morphology and sizedistribution, and to control the chemical reactivity of the particles,full densification can be achieved at lower sintering temperatures. TheXRD patterns of the calcined nano-TCP powders are in good agreement withβ-TCP file (JC-PDS 9-169); the peaks are substantially broadened due tothe nanocrystalline nature of TCP.

Aging temperatures during precipitation affect the crystal growth ratewith room temperature and below being favored. Aging time affects theconversion of the chemical homogeneity, crystallite size, and particlemorphology and size distribution. pH affects the solubility of the TCPprecursor phase. For TCP synthesis, the preferred pH is above about 7and below about 10. Precursor addition rate affects the nucleation andcrystal growth rates and particle morphology. Fast addition rates arepreferred at both high and low precursor concentrations. Precursorconcentration affects the rate of reaction.

The nano-TCP precursor phase calcined at 600° C. gives an ultrafinecrystal size of about 50 nm, surface areas greater than about 150 m²/gand narrow particle size distributions with an average particle size of2.5 μm. The nano-TCP compact has superior sinterability when compared toconventional TCP. The highly densified TCP can be obtained bypressureless sintering at 1100° C.

Example 7 Effect of Synthesis Conditions on Calcination Temperature

Unlike hydroxyapatite, neither the α- nor β-phase of TCP is formed fromthe as-precipitated powders synthesized by the procedure and conditionsdescribed in Examples 1-6. The purpose of calcination is to first removeany volatile organics (e.g. alcohol, toluene, acetone) or inorganics(e.g. nitrates etc.) that are adsorbed and then to dehydroxylate the TCPprecursor phase (e.g. apatite, amorphous calcium phosphate, octacalciumphosphate) and crystallize it into α- and/or β-TCP.

To achieve these objectives, the TCP precursor powders are calcined inoxygen or other oxidizing atmosphere during the ramp to remove anyvolatile adsorbed species and under vacuum or other reducing atmosphere(e.g. nitrogen, argon or helium) while at the calcination (e.g., soak)temperature for a period of time to promote dehydroxlation andcrystallization of the appropriate TCP phase. To investigate the effectof calcination, the TCP precursor powders prepared in Examples 1-6 arecalcined at a soak temperature ranging from about 400° C. to 1400° C.for two hours.

At calcination temperatures typically from about 400° C. to 1000° C.,β-TCP (JC-PDS 9-169) is obtained. The lowest calcination temperaturerequired to form β-TCP, 400° C., will be achieved with a CaN and NHPconcentrations of 0.15 M and 0.1 M, respectively, at an addition rate ofabout 250 ml/min, at an aging time of about 100 hours, at any agingtemperature of about 25° C. and at a pH of about 8.5. At about a 400° C.calcination temperature, the β-TCP powders will possess a crystal sizeof about 25 nm, a surface area of about 200 m²/g and a particle size ofabout 0.8 μm. At a calcination temperature of about 1000° C. or less,the β-TCP powders will possess a crystal size from about 100 nm, asurface area of about 80 m²/g and a particle size of about 3 μm.

When this precursor powder is calcined above about 1000° C. to 1400° C.,a phase pure metastable α-TCP can be obtained if the powders are rapidlyquenched to room temperature. At a calcination temperature of about1100° C. or greater, a mixed α/β-TCP will be obtained of which the α-TCPforms about 50 vol. %. The α-TCP phase will possess a crystal size ofabout 150 nm whereas the β-TCP will possess a crystal size of about 125nm. The composite powder will possess a surface area of about 60 m²/gand a particle size of about 5 μm. By calcination at a temperature ofabout 1200° C., a pure α-TCP phase can be obtained. The α-TCP phase willpossess a crystal size of about 200 nm, a surface area of about 40 m²/gand a particle size of about 7 μm. At a calcination temperature of about1400° C., a α-TCP powders having a crystal size of about 300 nm, asurface area of about 30 m²/g and a particle size of about 9 μm can beproduced. Alternatively, if the powders are slowly quenched or held atlower soak temperatures during cooling, a mixed α/β-phase is formed. Thesecondary soak temperature and time determine the relative amount ofβ-TCP formed and physical properties of the composite powder. Whenholding at a secondary calcination temperature between 600° C. to about800° C. for a period of 2 hours to about 24 hours, a secondary β-TCP canbe introduced to form a composite powder. At lower temperature andshorter times, a lower volume fraction of β-TCP is formed, typicallyfrom about 5 to 20 vol. %. At higher temperatures and longer times, ahigher volume fraction of β-TCP is formed, typically from about 50 to 75vol. %.

Example 8 Effect of Precursor and Solvent Environment on the CalcinationTemperature Required to Form TCP

Typically, TCP precursor powders are calcined to dehydroxylate andcrystallize the precursor powders into a TCP phase. Consequently,precursor reactants (calcium salt, phosphate salt, and acid or base) andsolvents that can reduce hydroxylation and water retention in theprecursor lead to lower calcination temperatures and nanocrystallinecrystallite sizes (<100 nm).

When precipitating TCP precursors in an aqueous solution, calciumnitrate and ammonium hydrogen phosphate are the preferred calcium andphosphate sources, respectively. The precursor precipitates are aged at25° C. for 12 hours and are collected, washed, milled, dried andcalcined at 600° C. This β-TCP powder will possess a crystal dimensionof less than about 50 nm, a surface area in excess of about 150 m²/g, anarrow particle size distribution with an average particle size of about0.9 micron, and a calcium to phosphate ratio of about 1.5.

When precipitating TCP precursors from a polar organic solvent, calciumalkoxides or calcium acetates are preferred as the calcium source andphosphoric acid or trialkylphosphates (e.g., tributylphosphate ortriethyl phosphate) are preferred as the phosphate source. The precursorprecipitates are aged at 25° C. for 12 hours and are collected, washed,milled, dried and calcined at 400° C. This β-TCP powder will possess acrystal dimension of less than about 30 nm, a surface area in excess ofabout 200 m²/g, a narrow particle size distribution with an averageparticle size of about 0.9 micron, and a calcium to phosphate ratio ofabout 1.5.

Alternatively, TCP precursors can be precipitated from solution of waterand a polar organic solvent. In this case, calcium alkoxides andtrialkylphosphates are the preferred calcium and phosphate sources,respectively. The precursor precipitates are aged at 25° C. for 12 hoursand are collected, washed, milled, dried and calcined at 400° C. Thisβ-TCP powder will possess a crystal dimension of less than about 30 nm,a surface area in excess of about 200 m²/g, a narrow particle sizedistribution with an average particle size of about 0.9 micron and acalcium to phosphate ratio of about 1.5.

Example 9 Pressureless Sintering of TCP Powders

β-TCP is calcined at a temperature of about 600° C. in an oxygen/vacuumatmosphere, then uniaxially pressed in steel die to a pressure of about150 MPa, cold isostatically pressed (CIPed) to a pressure of about 300MPa, and finally pressurelessly sintered at a temperature of about 800°C. to about 1500° C. in an oxygen atmosphere for about 2 hours at a ramprate of 5° C./min.

Prior to sintering, the β-TCP powders can be unaxially pressed in asteel die at a pressure ranging from about 50 MPa to about 1 GPa withoutdetrimentally affecting the sintering process. After unaxially pressing,these compacts can be CIPed at a pressure ranging from about 50 MPa tothe maximum allowable pressure for the particular cold isostatic press.Alternatively, the TCP powder can be poured into a rubber mold withoutuniaxial compaction and then CIPed.

β-TCP is formed at sintering temperatures ranging from about 800° C. toabout 1100° C. At sintering temperatures greater than about 1100° C., amixed α/β-TCP material is formed whereas at sintering temperaturesgreater than about 1200° C., a pure α-phase is formed. Sinteringtemperatures ranging from about 900° C. to about 1100° C. result intheoretical densities greater than about 95% while sinteringtemperatures ranging from about 1000° C. to about 1100° C. result intheoretical densities greater than about 97% and a crystal size of lessthan about 500 nm. A high-density α-TCP also can be obtained atsintering temperatures greater than about 1200° C. with crystal sizeslarger than about 1 μM. Similar to Example 7, a dense α/β-TCP compositematerial can be obtained by holding α-TCP powder at lower secondary soaktemperature to reintroduce the β-TCP phase. The combination of soaktemperature and soak time can be used to control the volume fraction andcrystal size of β-TCP formed. Higher soak temperatures will result in alarger volume fraction and crystal sizes of β-TCP while lower soaktemperatures will results smaller volume fractions and crystal sizes.

The fracture toughness of the articles sintered by pressurelesssintering is measured by an indentation technique. The fracturetoughness is less than about 1 to 2 MPa·m^(1/2). Furthermore, bendingstrengths and equibiaxial flexure strengths are from about 100 MPa toabout 250 MPa. The compressive strength is about 500 MPa or greater.

Example 10 Hot Pressing of TCP Powders

TCP powders are hot pressed at a pressure of about 50 MPa or higher, ata ramp rate of about 5° C./min, and with a dwell time of about 30minutes at sintering temperature between about 700° C. and about 1300°C. in an oxygen, hydrogen nitrogen, argon, helium or vacuum atmosphere.Compared to the pressureless sintering process described in Example 9,the addition of a uniaxial pressure during sintering enhances thesintering process by reducing the sintering temperature at which fullydensified articles are achieved by several hundred degrees. Furthermore,the reduction of the sintering temperature results in minimized graingrowth as well.

Fully densified articles of β-TCP with a crystal size of less than about500 nm will be obtained by hot pressing at sintering temperatures lessthan about 1000° C. Similar to Example 9, higher sintering temperatureswill result in a fully dense α-TCP while lower secondary soaktemperatures after sintering can reintroduce the β-TCP. Furthermore,these densified articles are optically transparent. The application ofthe uniaxial pressure removes many pores, which are not removed bypressureless sintering. Fracture toughness measurements via indentationshow that fracture toughness is increased to between about 1.5MPa·m^(1/2) and about 3.0 MPa·m^(1/2). Furthermore, bending strengthsand equibiaxial flexure strengths are from about 150 MPa to about 400MPa. The compressive strength is about 700 MPa or greater. Finally,densified articles prepared by hot pressing possess better reliabilitythan articles prepared by pressureless sintering.

Example 11 Net Shape Forming and Hot Isostatic Pressing of TCP Powders

Geometrically complex monoliths comprised of TCP are processed in twosteps. First, the TCP powder is net shape formed into a green body andsecondly, the green body is densified by hot isostatic sintering.Microstructural and mechanical properties analysis can be used toevaluate the process for the net shape forming of geometrically complexTCP monoliths.

Complex shapes are formed by one of four processes. These processesinclude dry powder compaction, plastic flow, fluid removal, andgelation. Dry powder compaction can be carried out by cold isostaticpressing in a mold to net shape or green machining of the coldisostatically pressed green body. Examples of plastic flow processesinclude injection molding and extrusion/green machining. Examples offluid removal processes include slip casting and pressure casting/greenmachining. Gelation can be carried out by in situ polymerization andgelation using any combination of the following monomer/polymer systems:acrylamides, methacrylates, starches, sugars, alginates, chitosans, orcelluloses. Depending on the mold design, the green bodies can becylinders, tapered pins, blocks, or plates. In addition, the mold designcan introduce threading and cannulation. Nanoporosity (˜100 nm) can beintroduced by changing the morphology of the nanocrystals whereasmacroporosity (˜150 μm) can be introduced using a polymer spheres thatcan be removed prior to hot isostatic pressing. To obtain a green partthat can be fully densified and possess strength sufficient for astructural application, the green body should possess the followingproperties: (1) absence of inclusions or impurities, (2) absence ofregions of high or low density, and (3) small pore sizes and narrow poresize distribution.

Hot isostatic pressing (HIP) is dependent on the simultaneousapplication of high temperature and high pressure to densify the part.The advantage of this process is that it can sinter complex shapes andreduce sintering temperatures while decreasing the size of processingrelated bulk defects. After net shape forming, the green body is eithersintered to closed porosity (greater than 95% theoretical density) orencapsulated/vacuum sealed in glass or a metal and then hot isostaticpressed. Typically pressures during HIP of TCP are between about 50 MPaand the greatest operating pressure of the HIP system. Sinteringtemperatures occur between about 600° C. and about 1500° C. Soak timesrange from about 10 minutes to about 2 hours. Phase behavior of TCP issimilar to those observed in Examples 9 and 10.

TCP articles densified by HIP are typically fully dense (greater than97% theoretical dense), nanocrystalline (crystal sizes less than 250nm), and optically transparent. Because critical defect sizes arereduced by preserving nanocrystallinity, and process-related defectshave largely been removed through HIP, the fracture toughness andstrength of TCP articles is enhanced. For example, fracture toughnessesvia indentation testing between about 1.5 MPa·m^(1/2) and about 3.0MPa·m^(1/2) can be obtained. Furthermore, bending and equibiaxialflexure strength from about 150 MPa to about 400 MPa can be obtained.Compressive strengths greater than about 750 MPa have also can beobtained. Finally, articles produced by HIP possess better reliabilitythan articles prepared by hot pressing.

Example 12 Resorption and Bioactivity of TCP

When comparing like powders, coating, porous bodies or dense articles ofTCP, the degree of protein adsorption, cell attachment, adhesion,proliferation, and matrix synthesis is a function of the crystal size.For example, protein adsorption, cell attachment, adhesion,proliferation, and matrix synthesis are enhanced for TCP materialshaving smaller crystal size compared to those with larger crystal size.Accordingly, nanocrystalline TCP having a crystal size or crystal sizeranging from about 20 to about 200 nm are preferred for applicationsrequiring high bioactivity. Resorption of TCP into the body also isfound to be a function of crystal size when all other properties forpowders, cement, pastes, void fillers, coatings, porous bodies and densearticles are similar. Typically, small grain materials resorb morerapidly than coarse grain materials. Using the method of the invention,the TCP crystal and grain sizes can be varied to control the resorptiontime of these materials when used as a powders, cement, pastes, voidfillers, coatings, porous bodies and dense articles.

Example 13 TCP Nanocomposites

To further increase the fracture toughness of TCP materials, a porous ordense TCP composite can be formed. In one method, a secondary additivepossessing material properties different than those of TCP can beincorporated into the microstructure. Desirably, the secondary additiveis stronger than the TCP material, for example, the secondary additivetypically possesses a higher fracture toughness, hardness, ductility,and/or strength than the TCP material. The secondary additive can beselected from the group consisting of alumina, titania, zirconia, gold,silver, titanium, nitinol, and combinations thereof. The fracturetoughness of the TCP material also can be increased by introducing asecondary additive possessing a non-spherical aspect ratio (e.g., anaspect ratio greater than about 1.5, or about 2). Such secondaryadditives having non-spherical aspect ratios include, for example,alumina, hydroxyapatite, titania, zirconia, or metallic needles, rods orwhiskers or carbon nanotubes, plates, and the like. The secondaryadditive can have a length on the order of a nanometer to severalmicrons, but should be small enough so as to be easily dispersed duringsynthesis or as to create processing defects that cannot be removedthrough pressure-assisted densification.

To synthesize such a composite material, the secondary additive can behighly dispersed in either the calcium salt or ammonium salt solution,and should be present in the volume fraction that is desired in thefinal composite article. The TCP is then precipitated in the presence ofthe secondary additive so as to achieve a high dispersion of thesecondary additive in the TCP. It is preferred that the secondaryadditive be fully reacted and crystalline in order to minimize reactionsdue to pH or the presence of calcium or phosphate ions. The compositeprecipitate is recovered and processed as previous described. Whensintered to full density by methods described in Examples 9-11, themicrostructure of the composite material is such that the TCP phase isstill nanocrystalline whereas the secondary phase is highly dispersed.Because the secondary phase is highly dispersed, fracture toughness andstrengths enhancements are achieved with smaller volume fractions of thesecondary additive. The secondary phases preferably exist in domains ofabout 1 micron or less in the TCP matrix with the preferred secondaryphase dispersed as the finest individual elements in the matrix.Fracture toughness of greater than about 2 MPa·m^(1/2) and bending andequibiaxial flexure strengths of about 200 MPa or greater are easilyachieved.

Example 14 Calcium Phosphate-Polymer Nanocomposites

Unlike Example 13 where the objective was to improve the mechanicalproperties of TCP, the presence of calcium phosphates, such as apatitesor TCP, in polymer nanocomposites is used to both mechanically reinforcethe polymer by increasing stiffness and strength and to increase thebioactivity of the composite article. To achieve these requirements, thecalcium phosphate biomaterials desirably are highly dispersed in thepolymer phase. Preferably, the calcium phosphate exists in domains ofabout 1 micron or smaller. More preferably, the calcium phosphate existsin domains on nanometer scale (e.g., about 20 to about 500 nm). Tosynthesize such polymer nanocomposites, the calcium phosphatenanocrystals should be dispersed in the reaction medium containing themonomer prior to the synthesis of the polymer. Preferred polymersinclude polylactic acid, polyglycolic acid, polylactic acid/polyglycolicacid copolymers, polypropylenefumarate, polyhydroxybutyric acid,polyhydroxyvaleric acid, polycaprolactone, polyhydroxycarboxylic acids,polybutyrene succinate, polybutylene adipate, and collagen.

Example 15 Processing of Porous TCP

As previously discussed in Example 10, a porous TCP can be made bymolding TCP with polymer spheres having diameters ranging from about 25nm to about 300 microns. Once compacted, the polymer spheres are burnedout during a high temperature treatment (e.g., calcination or sintering)leaving extensive and interconnected porosity while the pore walls arecomprised of nanocrystalline TCP. By controlling the particle sizedistribution of polymer spheres, the pore size of the TCP across severalorders of magnitude can be introduced. The volume fraction is thenselected to achieve a particular strength. At high polymer sphere volumefractions, the compressive strength of the material is low. However,there is a critical volume fraction where these is an insufficientvolume fraction to form interconnected porous. As an alternative methodto forming porous bodies with nanocrystalline walls, a foaming agent maybe added to a highly loaded slurry of TCP to create pores. The pores arepreserved by the addition of a curing agent such as a polymer ormonomer. The slurry is then dried and fired to remove any organic. Thismethod also allows the ability of to form pores with a wide pore sizedistribution. Another method is to cast a highly loaded slurry with thecuring agent into a porous polymer foam. Once dried, the material isfired to remove the organics and polymer foam leaving a porous TCP.Pores smaller than 100 nm can be formed by introducing a surfactant(i.e. cetyl triammonium bromide) or triblock copolymer (i.e.PEO-PPO-PEO) micelles during the precipitation of the TCP precursor orinto the slurry. The micelle TCP solution is then processed with polymerspheres, foaming agents or polymer foams to create a porous body withpores ranging from 25 nm to 300 microns in size.

Example 16 Bicontinuous Biphasic Calcium Phosphate Composite

In this example, a dense article comprising of two bicontinuous phasesof nanocrystalline hydroxyapatite and TCP is proposed. The article wouldbe highly dense (greater than 95% of theoretical density of thecomposite) and possess a high bending strength. The bicontinuous phasesexist in channels with diameters ranging from 1 micron to 300 microns.Furthermore, the crystal sizes of each bicontinuous phase can beseparately changed from about 50 nm to about 5 microns.

Once implanted, the TCP phase will resorb at a rate determined by itsdensity, crystal size and channel diameter. As the TCP resorbs, a poroushydroxyapatite emerges from the dense bicontinuous biphasic calciumphosphate composite, and the host tissue begins to infiltrate the poroushydroxyapatite. The resorption rate of the hydroxyapatite is determinedby its density, crystal size and channel diameter.

To produce such an article, a porous TCP can be formed according to themethod of Example 15. After drying, a highly loaded slurry ofhydroxyapatite is poured into the porous TCP. Alternatively, a poroushydroxyapatite body can be formed according to Example 15 and theninfilitrated with a TCP slurry. This now bicontinuous biphasic calciumphosphate composite is then sintered according to methods described inExamples 8-10.

Example 17 Calcium Phosphate Structures as a Delivery Vehicle forPlasmid DNA, RNA, Proteins, and Drugs

The surfaces of a calcium phosphate powder such as TCP or hydroxyapatiteare saturated with plasmid DNA or RNA for gene delivery, proteins suchas bone morphogenetic proteins (BMPs), or drugs such as bisphosphonatesand antibiotics for drug delivery. Once the organic materials have beenfully adsorbed to the surface, the powders are recovered and dried.These powders can be used in pastes, cements, coatings, void fillers orimplants desiring gene or drug delivery. To form an implant structure,the powders can be CIPed at pressures of about 100 MPa or greater, attemperatures of about 25° C. or higher, and at times of about 5 minutesor greater. By CIPing, the density is increased thereby ensuringsustained delivery of the active agent. Furthermore, the high density ofthe article allows it to be used as a load-bearing implant.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A composition comprising particulate tricalcium phosphate (TCP)having an average particle size of about 5 μm or less, an averagecrystal size of about 250 nm or less and a surface area of about 20 m²/gor greater, wherein when the particulate TCP is densified to form anarticle having a minimum dimension of about 0.5 cm or greater thearticle transmits about 50% or more light having a wavelength in therange of about 150 nm to about 1,000 nm.
 2. The composition of claim 1,wherein the particulate TCP has an average particle size of about 1 μmor less.
 3. The composition of claim 1, wherein the particulate TCP hasan average crystal size of about 200 nm or less.
 4. The composition ofclaim 1, wherein the particulate TCP comprises α-TCP.
 5. The compositionof claim 1, wherein the particulate tricalcium phosphate is densified.6. The composition of claim 1, further comprising a secondary additive.7. The composition of claim 6, wherein the secondary additive is presentin an amount of between about 1% and about 50% by volume.
 8. Thecomposition of claim 6, wherein the secondary additive comprises astructural additive.
 9. The composition of claim 8, wherein thestructural additive comprises a metal oxide.
 10. The composition ofclaim 9, wherein the metal oxide comprises zirconia.
 11. The compositionof claim 8, wherein the structural additive has an aspect ratio of about2 or greater.
 12. The composition of claim 6, wherein the secondaryadditive is an organic species.
 13. The composition of claim 6, whereinthe secondary additive is a polymeric additive.
 14. The composition ofclaim 13, wherein the polymeric additive is selected from the groupconsisting of polylactic acid, polyglycolic acid,polylactic/polyglycolic acid copolymers, polypropylenefumarate,polyhydroxybutyric acid, polyhydroxyvaleric acid, polycaprolactone,polyhydroxycarboxylic acids, polybutyrene succinate, polybutyleneadipate, collagen, chitosan, alginate, celluloses, starches, sugars,polypeptides, polyethylene glycols, vinyl pyrrolidones, acrylamides,methacrylates, copolymer micelles, and combinations thereof.
 15. Thecomposition of claim 6, wherein the secondary additive is a biologicaladditive.
 16. The composition of claim 15, wherein the biologicaladditive is selected from the group consisting of plasmid DNA, RNA,proteins, bone morphogenetic proteins, and combinations thereof.
 17. Thecomposition of claim 6, wherein the secondary additive is apharmaceutical additive.
 18. The composition of claim 1, wherein whenthe particulate TCP is densified to form an article having a minimumdimension of about 0.5 cm or greater the article has a compressivestrength of 150 MPa or greater.
 19. The composition of claim 1, whereinwhen the particulate TCP is densified to form an article having aminimum dimension of about 0.5 cm or greater the article transmits about70% or more light having a wavelength in the range of about 150 nm toabout 1,000 nm.
 20. The composition of claim 1, wherein the particulateTCP comprises β-TCP.
 21. The composition of claim 1, wherein when theparticulate TCP is densified to form an article having a minimumdimension of about 0.5 cm or greater the article has a density that is90% of the theoretical density or greater.
 22. The composition of claim1, wherein the particulate TCP is produced using a wet chemicalapproach.
 23. The composition of claim 22, wherein the wet chemicalapproach comprises (i) precipitating a TCP precursor material from asolution containing a calcium salt and a phosphate source, (ii)recovering the TCP precursor material, (iii) milling the TCP precursormaterial to form a powder, and (iv) transforming the TCP precursorpowder to form particulate TCP.
 24. The composition of claim 23, whereinthe TCP precursor powder is transformed to particulate TCP bycalcination.
 25. A composition comprising particulate tricalciumphosphate (TCP) having an average particle size of about 5 μm or less,an average crystal size of about 250 nm or less and a surface area ofabout 20 m²/g or greater, wherein the particulate TCP is produced usinga wet chemical approach, and wherein when the particulate TCP isdensified to form an article having a minimum dimension of about 0.5 cmor greater the article has a density that is 90% of the theoreticaldensity or greater.
 26. The composition of claim 24, wherein the wetchemical approach comprises (i) precipitating a TCP precursor materialfrom a solution containing a calcium salt and a phosphate source, (ii)recovering the TCP precursor material, (iii) milling the TCP precursormaterial to form a powder, and (iv) transforming the TCP precursorpowder to form particulate TCP.
 27. The composition of claim 25, whereinthe TCP precursor powder is transformed to particulate TCP bycalcination.